Respiratory infection is a disease frequently encountered in clinical sites. There are two types of respiratory infection, namely, respiratory infection caused by viral infection and respiratory infection caused by bacterial infection. Antibiotics have no effects on viral respiratory infection. In principle, upon administration of antibiotics, a causative microorganism is first identified, and an antibiotic used therefor is then selected depending on the sensitivity of the causative microorganism to drugs, etc. However, since it takes a long period of time to search for such a causative microorganism by blood culture or examination of sputum, in reality, a treatment using antibiotics has been started before determination of a pathogen in almost all cases. Moreover, since the detection sensitivity of bacteria is not necessarily high in such blood culture or examination of sputum, even if the results are negative, bacterial infection cannot be denied. Thus, antibiotics are used even for patients with respiratory infection suspected to have bacterial infection. Hence, administration of antibiotics to patients who do not need them causes an increase in resistant bacteria (Non Patent Literature 1).
For the proper use of antibiotics, it is necessary that the presence or absence of bacterial infection in respiratory infection can be promptly and precisely diagnosed in clinical sites. As such a diagnostic marker, the usefulness of procalcitonin has been studied. It has been reported that when lower respiratory tract infection or pneumonia is treated with antibiotics, while using the concentration of procalcitonin in blood as an indication, the amounts of the antibiotics used can be decreased (Non Patent Literature 2 and Non Patent Literature 3).
Procalcitonin has also been used as a diagnostic marker for sepsis. In this case, procalcitonin is used as an indication, such that when the procalcitonin concentration in blood is 0.5 ng/mL or higher, the subject is determined to have sepsis, and when the procalcitonin concentration is 2.0 ng/mL or higher, the subject is determined to have severe sepsis. On the other hand, when procalcitonin is used as a marker for lower respiratory tract infection or pneumonia, a change in a blood procalcitonin concentration that ranges from 0.1 to 0.5 ng/mL must be detected, and thus, there is a restriction that a highly sensitive assay must be used. Moreover, it has also been reported that, since procalcitonin is stably present in blood, the blood procalcitonin concentration is relatively slowly lowered even in a case in which the treatment has been successful (Patent Literature 1). It is considered that when the half-life of procalcitonin in blood is long, a concentration change is hardly detected.
As a novel marker for sepsis that is superior to procalcitonin, the usefulness of sCD14-ST (soluble CD14 antigen subtype; alias name: presepsin) has been studied. It has been reported that even if such sCD14-ST is used in a comparison between a patient with sepsis and a patient with systemic inflammatory response syndrome (SIRS) that is difficult to be distinguished from sepsis, it shows a high value in the blood of the sepsis patient. Thus, it has been considered that sCD14-ST is useful as a diagnostic marker for sepsis (Non Patent Literature 4 and Patent Literature 2).
Furthermore, it has also been reported that sCD14-ST is generated in a process in which cells englobe and digest foreign microorganisms or foreign matters, and that an increase in the concentration of sCD14-ST in synovial fluid can be detected in a disease that involves phagocytosis attended with autoimmune response or infection in local sites, such as arthritis (Patent Literature 3).